Coil stops and plunger assemblies for flippers.

LTG : )

As an operator, I always have flipper rebuild parts on hand. A spare flipper assembly base (for both sides) as the screws that hold the coil stops like to snap off from time to time. Flipper buttons! Those things love to blow up at the worst possible time. Rubbers, LOTS of rubbers, and replacement bulbs.
Depending on how much, and how hard you play, I'd recommend having a few replacement parts for other things that can wear out, like sling shot switches, pop bumper parts, micro switches, and a set of opto's.

We are starting to see the switch actuator bracket (the thing with the white nylon piece) starting to fracture and break. So definitely at least get one left and one right plunger/link/crank assy if not full flipper rebuild kits. And lots of buttons!

Rod and Ring assy, and metal and fiber yokes for pop bumper assys.

Balls, light bulbs and nvram as its very easy on those 2 games.

something specifically for modern sterns -
the main thing I have break are those damn plastic things that push the EOS switch open, I've replaced at least 6 of those way before it was time to rebuild.